Field of the Invention
The invention relates to a signal processing system comprising a signal source, a signal processing unit and a connecting cable connectable between the source and the unit. The connecting cable may be selected from a plurality of connecting cables having different lengths with each having a variable cable loss which depends on the cable construction. The signal processing unit comprises a signal amplifier having automatic gain control, a low noise signal amplifier without automatic gain control and a switchable signal attenuator arranged in series. The signal attenuator has a change-over input coupled to a change-over circuit. The change-over circuit has a measuring signal input that is coupled to a signal detection circuit. An input of the signal detection circuit is coupled to a signal output for supplying a signal received from the connected cable. The supplied signal originating from the signal source has a substantially constant nominal amplitude.
Such a signal processing system is disclosed in U.S. Pat. No. 4,148,069. The signal source is a television camera connecting by a cable having a variable, optional length to a remote camera signal processing unit which may operate as a camera control unit. The processing unit applies control signals to the camera through the cables. The system may operate with time or frequency-division multiplex transmission. The cable loss or the cable attenuation of a signal to be transmitted of a predetermined frequency band is predominantly determined by the cable construction, such as the length of the cable and the diameter of the cable core or cable cores, depending on the chosen type, such as a coaxial or a triaxial cable. The cable loss varies as a function of the temperature variation at the cable. With the aid of the signal amplifier with automatic gain control, the influence of the variable cable loss can be compensated for. Therefore, the processing unit output signal has a substantially constant nominal amplitude, independant of the type of cable. In practice, it has been found that a cable loss variation from 0 to 52 dB may occur with a signal to be transmitted with a frequency of 45 MHz. As an example, a triaxial cable having a length between 0 and 2 kms and a diameter of 14 mm is convenient.
The use in the system of a signal amplifier having an automatic gain control between 0 and, for example, 52 dB over a signal frequency band from, for example, 5 to 50 MHz, is not attractive because, without special measures, the signal-to-noise ratio will be impermissibly low.
For that purpose the above-mentioned U.S. patent proposes to replace the single signal amplifier having the automatic gain control for the entire range of the cable loss by a combination of a signal amplifier having an automatic gain control over a smaller range, a signal-to-noise ratio which is consequently simpler to realize, and a switchable signal attenuator.
In practice it has been found that during television camera signal processing sudden amplitude changes may be produced. On display of the camera signal these amplitude changes may become visible in an annoying, impermissible way.